Phosphor particles are used in a variety of applications such as flat panel displays and decorations, cathode ray tubes, and fluorescent lighting fixtures. Luminescence or light emission by phosphor particles may be stimulated by application of various forms of energy including electric fields (electroluminescence). Electroluminescent ("EL") phosphors have significant commercial importance. The luminescent brightness of such phosphors and the "maintenance" of this brightness are two criteria typically used to characterize phosphor particles.
Luminescent brightness is typically reported as a quantity of light emitted by the subject phosphor when excited. Because of the sensitivity of phosphor emission brightness to varying conditions of excitement, it is often useful to report the brightness of phosphors as relative brightnesses rather than as absolute brightness. "Maintenance" refers to the rate at which phosphors lose brightness (i.e., decay) with operating time. The rate of decay is substantially increased if the phosphor particles are subjected to conditions of high humidity while being operated. This effect of moisture or high humidity has been referred to as "humidity-accelerated decay".
Particulate EL phosphors are most commonly used in thick film constructions. These devices typically include a layer of an organic material having a high dielectric constant and which forms a matrix for a load of phosphor particles. Such layers are typically coated on a plastic substrate having a transparent front electrode. A rear electrode is typically applied to the back side of the phosphor layer, with a dielectric layer sandwiched therebetween. When an electric field is applied across the electrodes, the proximate portions of the layer emit light as the phosphor particles therein are excited.
Organic matrices and substrate materials, as well as organic coatings applied to individual particles, have typically been ineffective in preventing the decay of brightness caused by the diffusion of water vapor to the phosphor particles. For this reason, thick film electroluminescent devices have been encased in relatively thick envelopes, e.g., 25 to 125 microns, of moisture-resistant materials such as fluorochlorocarbon polymers (e.g., ACLAR Polymers from Allied Chemical). However, such envelopes are typically expensive, result in unlit borders, and have the potential of delaminating, for example, under heat.
To improve their moisture resistance, phosphor particles have been encapsulated in an inorganic coating, such as a coating of one or two oxides. Inorganic coating techniques have been employed with varying degrees of success. Hydrolysis-based processes for encapsulating EL phosphor particles in an inorganic coating, e.g., hydrolysis-based chemical vapor deposition (CVD), have typically been the most successful. In hydrolysis-based CVD processes, water and oxide precursors are used to form the protective coating. Such hydrolysis-based CVD processes have been able to produce moisture insensitive encapsulated phosphor particles, while minimizing process related phosphor damage and retaining a high initial luminescent brightness. One such coating which has been considered desirable for encapsulating EL phosphors is a coating of aluminum oxide produced by a hydrolysis-based CVD process. The use of aluminum oxide to so coat phosphor particles has been found desirable, at least in part, because reactive, volatile precursors exist which can readily form aluminum oxide coatings exhibiting desirable optical, electrical and moisture protective properties.
Phosphor particles with such aluminum oxide coatings have been fabricated which exhibit high brightness and moisture insensitivity (i.e., the phosphor particle is protected to a certain degree from moisture in vapor form). However, amorphous and/or low temperature derived aluminum oxide coatings, such as those that have typically been produced by hydrolysis-based CVD processes, are susceptible to exhibiting undesirably low chemical durability against exposure to condensed moisture or otherwise liquid water. Such low chemical durability can preclude the use of such aluminum oxide coatings with aqueous polymer binder systems, can result in weak interfaces between the phosphor particle and the polymer matrix and/or can provide inadequate protection in condensing atmosphere conditions.
Therefore, there is a need for an aluminum oxide phosphor coating, such as the amorphous and/or low temperature derived aluminum oxide coatings produced by hydrolysis based CVD, which provides encapsulated EL phosphor particles, or phosphors in other forms, which exhibit high initial brightness, extended retained brightness (even in high humidity environments) and greater resistance to corrosion (i.e., chemical degradation) caused by exposure to condensed moisture or otherwise liquid water.